The radio frequency (RF) spectrum is a limited commodity. Only a small portion of the spectrum can be assigned to each communications industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum. Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
Wireless service providers also seek other ways of using the available spectrum as efficiently as possible. One important technique for maximizing spectral efficiency is to minimize overhead message traffic. If the number of overhead messages transmitted is reduced, fewer overhead channels are required to carry overhead messages. This frees up spectrum for user traffic. Also, reducing the number of overhead messages reduces the processing load in both the mobile stations and the base stations of the wireless network.
As is well known, when a wireless mobile station moves from a cell that is served by a source base station to a cell that is served by a target base station it becomes necessary to transfer or “hand off” the wireless mobile station from the source base station to the target base station. In most wireless networks approximately forty percent (40%) to fifty percent (50%) of all active calls experience some type of handoff. These handoffs involve adding cells or dropping cells to an active call, or handing the call over to another cell under the control of another base station. In either case a decision must be made prior to the handoff identifying which cell or cells are to be involved in the handoff. If the cell selection algorithm that is employed is not robust, unnecessary air signaling and call quality degradation may result. Improvements to cell selection algorithms for handoffs can result in a lower call drop rate and better overall network performance.
Prior art cell selection algorithms are based solely on the power levels of neighboring base stations. The wireless mobile station makes pilot signal strength measurements of the candidate base stations and uses the measurements to determine which cell (of a number of candidate cells) is to receive the handoff. This approach can lead to inaccurate decisions if the radio frequency (RF) environment is less than ideal. Specifically, phenomena such as fading, additive combining, and dopplers can produce misleading pilot strength signal readings at the wireless mobile station. Erroneous pilot strength signal readings may cause the base station selection algorithm to select the wrong handoff base station.
There is therefore a need in the art for an improved apparatus and method to enable a wireless mobile station to select an optimal handoff base station (as a target base station) for the handoff of the wireless mobile station from a source base station to the target base station. There is also a need in the art for an improved apparatus and method to provide an improved handoff decision algorithm in a wireless mobile station.